1. Field of the Invention
The present invention relates to spring brake systems for heavy duty vehicles, and in particular to a removable, breathable filter plug for attachment to the service brake housing or emergency brake housing of a spring brake actuator which prevents moisture and unwanted contaminants from entering the actuator, while allowing air to pass into and out of the housing during normal use.
2. Description of the Prior Art
Spring brake actuators are widely used in the trucking industry to control the brakes of heavy-duty vehicles and their trailers. There are two basic types of spring brake actuators, service brake actuators, and emergency brake actuators. These actuators can be deployed as separate assemblies, or combined into a single dual assembly including both a service brake and an emergency brake actuator. A service brake actuator is operator controlled and is used for slowing or stopping a moving vehicle. An emergency or parking brake actuator is typically held off with air pressure while the vehicle is in use, but automatically engages (thereby applying the brakes) when air pressure is removed, such as when the vehicle is turned off or if there is a failure in the vehicle air system.
A typical service brake actuator is characterized by a closed housing which contains a movable diaphragm stretched across the inside. One side of the diaphragm moves a slidable push rod which extends out of the housing for attachment to the brakes of the vehicle. On the other side of the diaphragm a sealed chamber is formed within the housing. An opening is provided in this chamber for connection to a pneumatic (air) pressure source usually provided by an air compressor on board the vehicle. The brakes of the vehicle can be applied by introducing sufficient pneumatic pressure into the sealed chamber to act against the service brake diaphragm which moves the push rod out. A small return spring is ordinarily provided inside the service brake housing around the push rod to urge it to retract when the air pressure behind the diaphragm is reduced.
While the vehicle is operating, air pressure is being constantly applied to and removed from the sealed chamber in the upper portion of the service brake housing to operate the brakes of the vehicle. The application and removal of air pressure causes the diaphragm inside the housing to move back and forth, moving the push rod in and out. In order for the diaphragm to be able to expand, the air in the lower portion of the service brake housing must be allowed to escape to the outside. Then, in order for the diaphragm to contract, air must be allowed to enter this area.
Typical service brake housing assemblies include a plurality of air holes in the lower housing to allow air to enter and exit this region as the diaphragm moves back and forth. The unfortunate effect of this action is to allow dirt, water, oil, debris, and other unwanted materials to enter the service brake housing from the outside through the air holes.
A typical emergency brake actuator is attached in axial alignment with or made a part of the service brake assembly. The emergency brake is a separate closed housing which contains a heavy main compression spring and a second movable diaphragm creating a second sealed chamber. The emergency brake diaphragm is also attached to or directly associated with the slidable central push rod of the service brake. The second sealed chamber is formed inside the emergency brake housing on one side of the diaphragm, and the heavy main compression spring is deployed on the opposite side. As with the service brake, the sealed chamber of the emergency brake is connected to the on-board pneumatic source of the vehicle. As long as sufficient air pressure is provided to the sealed chamber, the diaphragm in the emergency brake will remain fully expanded thereby compressing (caging) the main spring. However, should air pressure fall, or should there be a leak in the sealed chamber, the diaphragm will be unable to hold the main compression spring in place. When this occurs, either slowly or quickly, the main compression spring will expand causing the push rod to be extended out thereby applying the brakes of the vehicle.
Under normal conditions, when the vehicle is parked, the air pressure to the emergency brake is removed causing the main compression spring to apply the brakes. As the emergency brake is engaged and disengaged, air pressure is introduced and removed from the sealed chamber in the lower portion of the emergency brake housing, causing the diaphragm inside the housing to move back and forth. In order for the diaphragm to expand and compress the main spring, the air in the upper portion of the housing around the spring must be allowed to escape to the outside. Then, in order for the diaphragm to contract and release the main spring, air must be allowed to enter the area of the upper housing around the spring.
Typical emergency brake housing assemblies also include a plurality of air holes in the upper housing in the vicinity of the main spring to allow air to enter and exit this region as the diaphragm moves back and forth. The unfortunate effect of this action is the same as in the service brake housing, allowing dirt, water, oil, debris, and other unwanted materials to enter the emergency brake housing from the outside through the air holes.
Several inadequate or cumbersome solutions have been advanced to respond to this problem. The inventions of U.S. Pat. Nos. 4,508,018 and 5,671,654 describe a sealed internal breathing system which utilizes a tubular structure connecting between the service brake housing and the emergency brake housing. Air is transmitted between the two sealed housings through the tubular structure, so that no outside air is ever needed. However, the use of such a tube requires specially designed housings for both the spring brake and emergency brake making it difficult and costly to manufacture.
The invention of U.S. Pat. No. 5,320,026 employs a rubber dust guard in the upper housing between the power spring and the housing wall to prevent particulate matter from passing beyond the spring into the actuator tube where it might affect the sliding rod. However, this invention does not prevent foreign matter from entering the upper chamber in the first place through the air openings. Thus, foreign material can collect around the power spring potentially affecting operation of the brake.
The inventions of U.S. Pat. Nos. 5,372,059 and 5,722,311 use a pair of valves: a first one-way valve allows air to enter the area around the main spring only from the sealed chamber in the spring brake housing; another other one-way valve only allows air to escape from this area to the outside. A similar approach is taught in U.S. Pat. No. 5,873,297 which employs a double check valve in the housing, and a separate valve in the upper housing diaphragm for communication between the sealed chamber and the area around the main spring. Each of these inventions requires cumbersome specially designed emergency brake housing structures including at least one opening(s) in the spring brake diaphragm, together with the use of no less than two separate valves, leaving such systems vulnerable to several different kinds of failures which could be costly and difficult to repair.
The invention of U.S. Pat. No. 5,836,233 describes a spring brake actuator which employs breather holes located on the cylindrical side walls of the upper housing such that when the main spring is compressed or caged, the diaphragm covers the holes. However, when uncaged, these unfiltered breather holes are exposed to the atmosphere, allowing foreign material to enter the housing. A check valve is also provided in a cap attached to the upper housing.
The invention of U.S. Pat. No. 5,937,733 describes a spring brake actuator having a filter integrated into the emergency brake housing. A series of vent openings, valves, seals and internal conduits allow air to escape from both the service and emergency brake housings through this filter. Another similar device utilizes a hydrophobic filter system using a Gore-Tex membrane which is integrated into the top of the emergency brake housing to prevent water and contaminants from entering the housing while allowing air to flow in and out. However, because this filter system is integrated into a caging device (used to manually compress the main spring) it is impossible to remove or change the filter should it become clogged. The integral caging device is bulky and requires more space under the vehicle than a brake without such a device, thereby making such a brake assembly unusable on vehicles with restricted available space. In addition, although a relatively large filter is used, only a small area of the filter itself is ever utilized.
The present invention overcomes the drawbacks of the prior art while still enjoying the benefits of a breathable housing for a spring or service brake actuator by providing an easily removable and replaceable filter unit or plug for attachment to at least one of the breather holes of a spring or service brake assembly housing. Any remaining breather holes which are not used to receive a filter unit of the present invention are closed with plugs or seals, so that any air passing into or out of the breather holes passes must do so through filter units of the present invention. The filter unit of the present invention may be employed in the breather holes of the service brake housing, the emergency brake housing, or both. The present invention may be used in single diaphragm, single piston, piston and diaphragm, or dual diaphragm brake actuators. The filter unit includes a membrane through which air is allowed to pass into and out of the spring or service brake housing during normal use. The membrane itself is made of a material which prevents outside contaminants such as oil, water and debris from entering the housing. The membrane may be made of oilophobic and/or hydrophobic material.
The filter of the present invention may be easily inserted into any of the breather holes of either the service brake or emergency brake housing, the remaining holes being plugged closed. The filter may be installed in as few as one, or as many as all of the breather holes, depending on the desired availability of filtered air. The filter unit of the present invention has a very low (flattened) profile so that it takes up a minimal amount of space on the outside of the spring or service brake housing. It is made in a variety of different sizes and shapes to accommodate different sizes of breather holes and the housings of different brake actuators. The preferred embodiment of the present invention may be easily removed and replaced. As a result, the present invention may be used on any of a large number of existing spring or service brake housing assemblies at minimal cost, thereby avoiding the need for such costly, cumbersome and unnecessary structures as valves, tubes, conduits, perforated diaphragms and/or integrated caging apparatus in the brake actuator.
Several alternative designs for the filter plug of the present invention are available. In the preferred design, the unit includes a small outwardly protruding hollow shaft (made of ABS or some other durable material such as plastic, rubber or metal) having an annular flange at the end thereof which snaps into a breather hole of the brake housing assembly. The hollow shaft is attached to a low-profile structure which supports the filter. This structure includes a support base having an open central area that is in communication with the hollow interior of the shaft, a membrane stretched across the central area for filtering out foreign material, and a cover having a large open area above the membrane with ventilation holes to the exterior.
Alternative designs for the present invention include: (a) the filter being integrated into the plug as a single unit; (b) the filter being screwed over external threads in the support structure; or (c) the filter being screwed into internal threads in the support structure. In another alternative design, the annular flange at the end of the shaft is removed and replaced with helical threads for threadable insertion into a breather hole of a brake actuator housing. In another alternative design, the hollow shaft is eliminated, and the support base of the invention is provided with a wide opening directly to the exterior. This opening is glued, sealed or otherwise adhered or attached directly over a breather hole of the brake actuator housing.
It is therefore a primary object of the present invention to provide a removable breathable filter unit for attachment to at least one of the breather holes of a spring or service brake actuator which prevents foreign material from being introduced into the actuator from the outside.
It is also a primary object of the present invention to provide a removable breathable filter unit that may be attached to at least one of the breather holes of the service brake housing of a spring brake actuator.
It is also a primary object of the present invention to provide a removable breathable filter unit that may be attached to at least one of the breather holes of the emergency brake housing of a spring brake actuator.
It is a further important object of the present invention to provide a low cost removable breathable filter unit for attachment to the breather holes of any of a number of spring or service brake actuator designs without any modification to the actuators themselves.
It is a further object of the present invention to provide a breathable filter unit for attachment to the upper housing of an emergency spring brake actuator in conjunction with sealing plugs that are inserted into the remaining breather holes of the housing.
It is a further object of the present invention to provide a breathable filter unit for attachment to the lower housing of an service brake actuator in conjunction with sealing plugs that are inserted into the remaining breather holes of the housing.
It is another object of the present invention to provide a breathable filter unit for sealing attachment over one or more of the breather holes of a spring or service brake actuator housing.
It is another object of the present invention to provide a breathable filter unit for sealing attachment over one or more of the breather holes of either a single diaphragm, single piston, piston and diaphragm, dual piston or dual diaphragm spring or service brake actuator housing.
It is another object of the present invention to provide a spring or service brake housing having a lesser number of larger breather holes for use exclusively in conjunction with filter units of the present invention.
Additional objects of the invention will be apparent from the detailed descriptions and the claims herein.